1. Put the studentsí bridges and their application of bridge-building techniques to the test
2. Use math to convert pounds to grams and to develop a system to compare the strength to weight ratio of the bridges
3. Conclude the bridge lesson and bridge-building sessions with educational entertainment

Materials:

Each of the studentsí bridge-building teamsí bridges
Two flat, stable surfaces with a 12-inch clearance between them (we used desks)
A table
Sand
A bucket to hang sand from the bridge
A rope to hang the bucket of sand from the bridge
A hollow rod to disperse the weight of the rope, bucket and sand on the bridge
evenly
A small, sensitive scale (to measure the mass of bridges, which were 20-50 g)
A bigger scale (to measure the mass of sand, which was 3-15 lbs.)
Wax Paper

Introduction:

Talk about different types of bridges and the forces that act upon them. Use worksheets to further comprehension.

Development:

This activity was the culmination of a several-week-long lesson on bridges and the forces acting upon them. The 3rd graders had been using the last 15-30 minutes of our visits to construct their glue and toothpick beam and truss bridges from their drawn blueprints. When each bridge-building team completed construction, the bridge was measured in grams.
On a table, the two desks were set up with exactly 12 inches between them (the bridge contest rules stipulated that their bridge span at least a foot). Each bridge was placed across the divide. The rope was run through the rod and the rod was placed perpendicular to and on top of the center of the bridge beam. The rope was then fastened below to the empty bucket. Sand was gently added to the bucket until the bridge broke and the bucket fell onto the table. The bucket of sand was measured in pounds and recorded in a prepared chart on the board. The pounds value was first converted to that of grams, and then the mass of the sand was divided by the mass of the bridge to get the weighted strength value for each bridge.

Practice:

Divide class into groups. Each group designs a blue print of a bridge. Once a blueprint is approved, students begin building the bridges. Adjustments may be made.

Closure:

Not only was this a fun way to conclude the lesson on bridges, but it also provided a hands-on way for the students to manifest their learning in the real world and compete with other groups to test their reasoning. In the spirit of the table party game Jenga, each collapse followed a palpable buildup and ended with climactic excitement. It was entertaining and rewarding to let the kids see with their own eyes how much weight their bridges could support, and how continually added weight caused the structures to bend, twist and eventually snap.

Evaluation:

The main drawback with this activity is the extended time period it takes to design and build the bridges, but 20 minutes a day for a few weeks will seem unobtrusive and is worth the patience.